The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Shallow trench isolation (STI) provides electrical isolation between individual transistor devices in integrated circuits (ICs). STIs include trenches that are filled with high quality silicon (Si) oxide film. In some applications, an aspect ratio (AR) of the trench can be as high as 8:1 and an opening of the trench may narrow down to about 20 nm. Achieving void free STI fill is important because the film may be subjected to further processing in subsequent integration steps, which can expose the void. In some examples, the void may then be unintentionally filled with conductive material, which can lead to short circuits between different conductors on the chip.
In some applications, STIs are filled with film using high density plasma chemical vapor deposition (HDPCVD). However, for trenches with an AR that is higher than 4:1, it becomes very challenging for HDPCVD oxide to fill the STI without voids, even using a deposition-etch-deposition cyclic process.
Referring now to FIGS. 1A-1D, an example of gapfill using HDPCVD is shown. In FIG. 1A, a substrate 100 includes a trench 102 having sidewalls 104 and a bottom 106. In FIG. 1B, an oxide layer 112 such as SiO2 is deposited. The oxide layer 112 does not typically have uniform thickness. A cusp 114 usually develops in the trench opening. The oxide layer 112 is usually thinner at lower portions 120 of sidewalls 104 as compared to other locations such as a trench bottom 122 and field region. After additional HDPCVD steps in FIGS. 1C and 1D, the cusps 114 meet at the trench opening and a void 130 is created. The void 130 causes problems during subsequent processing.
While emerging flowable oxide methods provide liquid-like filling behavior, the requirement of flowability limits the achievable film density. Although post-deposition densification methods are available at extra cost, these methods have not proven successful. The alternative methods are not able to densify the film in a high aspect ratio structure due to the constraint of the surrounding sidewalls, which prevents the shrinkage required to fully densify the flowable oxide film.
ALD oxide may be used to gapfill deep trenches with high quality Si oxide film. However, a seam usually remains at a center of the trench after the film that is deposited on the side walls merges. Referring now to FIGS. 2A-2D, an example of gapfill using ALD is shown. In FIG. 2A, a substrate 200 includes a trench 202 having sidewalls 204 and a bottom 206. In FIG. 2B, an oxide layer 212 such as SiO2 is deposited during a first ALD cycle. The oxide layer 212 is conformal as can be seen on sidewalls at 220 and at a trench bottom 222. However, after additional ALD cycles in FIGS. 2C and 2D, a seam 230 is created. The seam 230 tends to have a high etch rate during subsequent wet chemical processing.